1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device of an in-cell touch-panel type, and more particularly, to an LCD device capable of securing a sufficient charge time and a sufficient sensor access time for a liquid crystal cell.
2. Discussion of the Related Art
Typically, liquid crystal display (LCD) devices display an image by controlling the light transmittance of a liquid crystal layer, using an electric field applied to the liquid crystal layer in accordance with a video signal. Such an LCD device has advantages of compactness, thinness, and low power consumption. By virtue of such advantages, the LCD device is used for a portable computer such as a notebook PC, an office automation appliance, an audio/video appliance, etc. In particular, an active matrix type LCD device, in which a switching element is provided for each liquid crystal cell, is advantageous in displaying moving pictures because it is possible to control the switching element in an active manner.
A thin film transistor (TFT) is mainly used as the switching element in the active matrix type LCD device.
Recently, a technique for attaching a touch screen panel to an LCD device as mentioned above has been proposed. The touch screen panel generally means a user interface, which is attached to a display device, to sense a touch point varying in electrical characteristics when an opaque object such as a finger or a pen comes into contact with the touch point. The LCD device, to which the touch screen panel is attached, can implement various applications, based on contact position information detected when a user's finger or a pen comes into contact with the screen of the touch screen panel.
However, such an LCD device has various problems such as an increase in costs caused by the touch screen panel, a reduction in productivity caused by an addition of a process for attaching the touch screen panel to an LCD panel, and a degradation in the brightness of the LCD panel, and an increase in the thickness of the LCD panel.
In order to solve the above-mentioned problems, an in-cell touch-panel system has been proposed. In the in-cell touch-panel system, no touch screen panel is used. In place, a touch sensor circuit including a sensor TFT is formed in each pixel cell of the LCD device.
FIG. 1 is a diagram showing timing of a data enable signal used in a related art in-cell touch-panel type LCD device. In the in-cell touch-panel type LCD device, each pixel cell thereof includes a pixel circuit for displaying an image, and a touch sensor circuit for sensing light from the outside of, and incident on, the pixel cell, and supplying a light sensing signal to a read-out integrated circuit, based on the sensed light. The pixel circuit and touch sensor circuit are connected in common to one gate line. Accordingly, the related art in-cell touch panel type LCD device may not be able to simultaneously display an image through the pixel circuit and; read the light sensing signal through the touch sensor circuit. To this end, in the related art LCD device, the image display time is reduced so that the operation for reading the light sensing signal can be carried out during a reserved time generated due to the reduced display time.
In order to reduce the image display time in the related art in-cell touch-panel type LCD device, it may be necessary to increase the time taken to read image data input from the outside of the pixel cell, and to output the read image data. Accordingly, it may be necessary to increase the frequency of the data enable signal functioning to control the output period of the image data.
To secure the time taken to vary the frequency of the data enable signal, the related art LCD device stores the image data in a memory, and controls the output period of the image data stored in the memory when the frequency of the data enable signal varies, using the varied frequency of the data enable signal.
In FIG. 1, “I-DE” represents a first data enable signal, namely, an original data enable signal input from the system, and “O-DE” represents a second data enable signal generated in accordance with a frequency modulation for the first data enable signal I-DE. FIG. 1 shows that the frequency of the second data enable signal O-DE is higher than that of the first data enable signal I-DE. The second data enable signal I-DE is output for a display time, and is not output for the remaining portion of the overall period, namely, a sensor access time.
In the above-mentioned related art case, however, there is a problem in that an increase in manufacture costs occurs because it is necessary to use the memory for storing image data, in order to generate the second data enable signal O-DE. Furthermore, since the second enable signal O-DE has an increased frequency, the display time is reduced. This causes a reduction in the charge time of the liquid crystal cell. As a result, degradation in display quality occurs.